Conventionally, an electric motor is known in which there are provided a first and a second rotors that are provided concentrically around an axis of rotation of an electric motor, and the relative positions in the circumferential direction of the first and second rotors, namely, the phase difference thereof are controlled in accordance with the rotational speed of the electric motor, or in accordance with the speed of a rotating magnetic field that is generated in a stator (e.g., refer to Patent Document 1).
In this electric motor, when the phase difference between the first rotor and the second rotor is controlled in accordance with the rotational speed of the electric motor, for example, the relative positions in the circumferential direction of the first and second rotors are altered via components that become displaced in a radial direction by the action of centrifugal force. If the phase difference between the first and second rotors is controlled in accordance, for example, with the speed of the rotating magnetic field that is generated in the stator, then the relative positions in the circumferential direction of the first and second rotors are altered by altering the rotating magnetic field speed by supplying a control current to the stator coil while each rotor is maintaining its rotational speed by inertia.
Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-204541